The present invention relates to projection television systems. Specifically, an aplanatic image combiner is described which provides for a curved entrance and exit surface for transmitting a curved image surface.
Projection television systems are known in the art. These systems have been developed to generate large screen images from a smaller cathode ray tube image generating surface. Conventional television signals in the raster scan format are demodulated and applied to three separate cathode ray tubes. Each of the cathode ray tubes is provided with a phosphor for generating one of the three primary colors red, green or blue. The images from these three cathode ray tubes are combined and projected through usually complex optics for enlargement in either a direct view or rear view projection system.
These systems have traditionally suffered from a loss of light output due to the projection, as well as loss in resolution. In order to preserve the image quality, complex and hence commercially unattractive projection lenses have been developed having achromatic, astigmatic and color correction elements.
Several efforts have been made to reduce the complexity of the projection lens used in these systems, thus lowering the cost of the projection system. In U.S. Pat. No. 4,249,205, a simplified projection lens is described which permits a single element objective lens with one aspheric surface to be used in connection with a prism which combines the light output from three cathode ray tubes. The prism receives an image from a concave image generating surface shown to be a cathode ray tube.
The presence of a plano exit surface for the prism described in the aforesaid U.S. patent results, however, in aberrations to what is essentially a curved image surface exiting the prism. The curved image plane, having been generated by the curved cathode ray tube's phosphor surfaces when exiting the plano surfaces exhibits field aberration as well as astigmatic and achromatic aberration. To correct for such aberrations, additional lens elements are usually required.
The present invention further takes advantage of the use of images having curved image surfaces. The present invention will provide for an aplanatic image generating source wherein the exiting image is maintained in its curved form for further magnification, with only a minimum of achromatic and astigmatic aberration which requires correction.